1. Field of the Invention
The present general inventive concept relates to a method and apparatus to enhance a low frequency component of an audio signal.
2. Description of the Related Art
It is difficult for small-sized speakers in portable devices, such as laptop computers and MP3 players, to completely reproduce a low frequency component of an audio signal due to physical limitations, i.e., small size. The difficulty of complete reproduction of a low frequency component of an audio signal may reduce the sound quality of the audio signal. Various methods have been suggested to address this side effect.
FIG. 1 is a block diagram of a conventional apparatus to enhance a low frequency component of an audio signal.
Referring to FIG. 1, the conventional apparatus includes low pass filters 110, a sine function generation module 122, a cosine function generation module 124, band pass filters 130, and mixers 140.
Once an audio signal is input to the conventional apparatus of FIG. 1, each of the low pass filters 110 extracts only a low frequency component (e.g. equal to or less than 120 Hz) by performing low pass filtering of the audio signal input with respect to a corresponding channel.
Both the sine function generation module 122 and the cosine function generation module 124 generate a harmonic signal by modulating the low-pass filtered signal.
The band pass filters 130 select only specific order harmonic signals by respectively performing band pass filtering of the signals modulated using a sine function and a cosine function.
Each of the mixers 140 generates an audio signal of a corresponding channel, in which a low frequency component is enhanced, by combining the input audio signal and the harmonic signal selected by a corresponding band pass filter 130.
As described above, a method of enhancing a low frequency component using a harmonic signal uses an acoustical effect that if human ears hear a tone of a frequency having multiples of a fundamental frequency, the person perceives the tone as if hearing a tone corresponding to the fundamental frequency.
FIG. 2 is a diagram illustrating ideal harmonic signals used to enhance a low frequency component of an audio signal.
Referring to FIG. 2, a fundamental frequency component of 220 Hz and harmonic signals are illustrated. As illustrated in FIG. 2, if a fundamental frequency component is 220 Hz, harmonic signals having multiples of 220 Hz, i.e., 440 Hz, 660 Hz, 880 Hz, and so on, are ideal harmonic signals to be used to enhance a low frequency component of an audio signal. Accordingly, the amplitude of each of the ideal harmonic signals decreases if their frequency increases as illustrated in FIG. 2.
If a person hears a tone of each of the ideal harmonic signals, the person perceives the tone as if hearing a tone corresponding to 220 Hz. Thus, using the ideal harmonic signals, the amplitude of a sound having a tone corresponding to 220 Hz seems to be enhanced.
However, conventional apparatuses to enhance a low frequency component of an audio signal cannot generate ideal harmonic signals as illustrated in FIG. 2.
FIG. 3 is a diagram illustrating harmonic signals generated using the conventional apparatus of FIG. 1.
FIG. 3 shows a fundamental frequency component that is 220 Hz and harmonic signals of 450 Hz, 650 Hz, and 900 Hz, wherein a modulation frequency is 50 Hz.
Referring to FIG. 3, since the modulation frequency used to generate harmonic signals is fixed to a pre-set frequency (e.g. 50 Hz), the conventional apparatus cannot generate the ideal harmonic signals of 440 Hz, 660 Hz, and 880 Hz illustrated in FIG. 2, but generates harmonic signals having errors, such as harmonic signals of 450 Hz, 650 Hz, and 900 Hz. In addition, unlike the ideal harmonic signals illustrated in FIG. 2 of which the amplitude decreases gradually, the harmonic signals illustrated in FIG. 3 maintain the same amplitude over all the frequencies.
As described above, a conventional method of enhancing a low frequency component of an audio signal has errors in terms of frequencies of harmonic signals as compared to ideal harmonic signals, and causes a severe variation in a tone since the same amplitude of harmonic signals is maintained instead of decreasing the amplitudes of the harmonic signals as the frequency of the harmonic signals are increased.